This invention relates to lenses. In particular, this invention relates to diffractive lenses. This invention also relates to lenses used in conjunction with magnetic, magneto-optic or phase change optical recording media. This invention also relates to methods for making lenses.
FIG. 1 illustrates a portion of a magneto-optic disk drive 2 comprising a magneto-optic recording disk 4 and a laser source 6 (typically a diode laser). During use, laser source 6 provides a laser beam 8 that pass through a beam splitter 9. Laser beam 8 then passes through and is focused by a diffractive lens 10 into a spot 11 on disk 4. Beam 8 is then reflected back through lens 10 to beam splitter 9. Beam 8 is then reflected by beam splitter 9 through a lens 12 which focuses beam 8 onto a sensor 14.
Unfortunately, during reading operations, a portion of laser beam 8 can reflect off of lens 10. This portion of laser beam 8 is then reflected by beam splitter 9 through lens 12 to sensor 14, where it introduces noise into the signal detected by sensor 14. We have experimented with tilting lens 10 as shown in FIG. 2, so that light reflecting off lens 10 does not interfere with the operation of sensor 12. However, we have found that if diffractive lens 10 is tilted, it is highly desirable to modify lens 10 so that it continues to form an aberration-free or substantially aberration-free focused spot on disk 4.
FIG. 3 schematically illustrates lens 10 in plan view. Lens 10 is arbitrarily drawn as a circle. Also shown in FIG. 3 are X and Y axes to facilitate a discussion of lens 10. Lenses such as lens 10 are commonly described using an equation as set forth below:                               P          ⁡                      (                          x              ,              y                        )                          =                              ∑                          i              =              1                        n                    ⁢                                                    α                i                            ⁡                              (                                                      x                    2                                    +                                      y                    2                                                  )                                      i                                              (        1        )            
where P is the phase profile (in radians) of the lens at a particular point x, y of the lens surface. In other words, light striking a point x, y of lens 10 has its phase modified by a number of radians equal to mod[2xcfx80]P(x,y) where mod[2xcfx80] is the modulus operator. In equation 1, the variable xcex1i is called the aspheric coefficient.
If lens 10 were a refractive lens, function P(x, y) would be proportional to the height profile of the lens. In other words,
H(x,y)=P(x,y)/xcexxe2x80x83xe2x80x83(2)
where H(x,y) is the height of the lens (in the z direction) at point x,y and xcex is the wavelength of light within the lens material. (See FIG. 3A, which illustrates a plano-convex refractive lens in cross section along lines Axe2x80x94A.)
As mentioned above, lens 10 is a diffractive lens. One type of diffractive lens is a blazed zone plate lens. FIG. 4 illustrates in cross section a blazed zone plate lens 10xe2x80x2. Blazed zone plate lenses can be used to focus light at a focal point, but as can be seen, they have a cross section that is somewhat different from a piano-convex refractive lens. In particular, lens 10xe2x80x2 comprises a set of concentric ridges and valleys. Blazed zone plate lenses are discussed in xe2x80x9cMicroopticsxe2x80x9d, by Sinzinger et al., published by Wiley-VCH Verlag GmbH in 1999, incorporated herein by reference.
Referring back to FIG. 1, for the case in which lens 10 is not tilted, lens 10 is designed such that xcex1i is not a function of x or y alone. Rather, (X is a function only x2+y2 (i.e. a function of the radial distance to the center of the lens). However, as mentioned above, if lens 10 is tilted about the x axis to prevent the above-mentioned reflection problem, lens 10 must be modified or it will no longer be able to narrowly focus laser light at spot 11. It would be desirable to modify lens 10 so that it can be tilted and still be able to narrowly focus light on spot 11 within the data recording disk.
A diffractive lens constructed in accordance with the invention comprises diffraction gratings. The spacing of the gratings varies over the lens surface so that the lens can focus light onto a small spot on a data recording medium even though the lens is tilted with respect to the laser beam. In one embodiments the lens is a blazed zone plate lens. In another embodiment, the lens is a phase zone plate lens. In another embodiment, the lens comprises alternating opaque and transparent regions. Because the spacing of the gratings is varied, any aberration in the focused spot of light provided by the lens reduced compared to what would be produced without varying the grating spacing. In one embodiment, the amount of aberration is substantially eliminated or completely eliminated.
In one embodiment, rather than having circular diffraction gratings, the gratings are somewhat oval shaped in order to reduce aberrations. For example, the gratings can have the shape of the intersection of a hollow cone and a slanted plane passing through the cone. However, as explained below, the gratings can have other shapes as well.